Nickel-Base Superalloy

ABSTRACT

A nickel-base superalloy is characterized by the following chemical composition (details in % by weight): 7.7-8.3 Cr, 5.0-5.25 Co, 2.0-2.1 Mo, 7.8-8.3 W, 5.8-6.1 Ta, 4.9-5.1 Al, 1.3-1.4 Ti, 0.11-0.15 Si, 0.11-0.15 Hf, 200-750 ppm C, 50-400 ppm B, 0.1-5 ppm S, 5-100 ppm Y and/or 5-100 ppm La, remainder Ni and production-related impurities. The alloy is distinguished by very good casting properties, a high resistance to oxidation, and good compatibility with TBC layers applied to its surface.

This application is a Continuation of, and claims priority under 35U.S.C. §120 to, International application number PCT/EP2005/055676,filed 1 Nov. 2005, and claims priority therethrough under 35 U.S.C. §119to Swiss application number 1897/04, filed 18 Nov. 2004, the entiretiesof which are incorporated by reference herein.

BACKGROUND

1. Field of Endeavor

The invention deals with the field of materials science. It relates to anickel-base superalloy, in particular for the production ofsingle-crystal components (SX alloy) or components with a directionallysolidified microstructure (DS alloy), such as for example blades orvanes for gas turbines. However, the alloy according to the inventioncan also be used for conventionally cast components.

2. Brief Description of the Related Art

Nickel-base superalloys of this type are known. Single-crystalcomponents produced from these alloys have a very good material strengthat high temperatures. This makes it possible, for example, to increasethe inlet temperature of gas turbines, which boosts the efficiency ofthe gas turbine.

Nickel-base superalloys for single-crystal components, as are known fromU.S. Pat. No. 4,643,782, EP 0 208 645 and U.S. Pat. No. 5,270,123, forthis purpose contain alloying elements which strengthen the solidsolution, for example Re, W, Mo, Co, Cr, as well as elements which formγ′ phases, for example Al, Ta, and Ti. The level of high-meltingalloying elements (W, Mo, Re) in the base matrix (austenitic γ phase)increases continuously with the increase in the temperature to which thealloy is exposed. For example, standard nickel-base superalloys forsingle crystals contain 6-8% of W, up to 6% of Re and up to 2% of Mo(details in % by weight). The alloys disclosed in the abovementioneddocuments have a high creep strength, good LCF (low cycle fatigue) andHCF (high cycle fatigue) properties and a high resistance to oxidation.

These known alloys were developed for aircraft turbines and weretherefore optimized for short-term and medium-term use, i.e., the loadduration is designed for up to 20,000 hours. By contrast, industrial gasturbine components have to be designed for a load time of up to 75,000hours.

By way of example, the alloy CMSX-4 from U.S. Pat. No. 4,643,782, whentested for use in a gas turbine at a temperature of over 1000° C., has aconsiderably coarsened γ′ phase after a load time of 300 hours, and thisphenomenon is disadvantageously associated with an increase in the creeprate of the alloy.

It is therefore necessary to improve the resistance of the known alloysto oxidation at very high temperatures.

A further problem of the known nickel-base superalloys, for example thealloys which are known from U.S. Pat. No. 5,435,861, is that in the caseof large components, e.g., gas turbine blades or vanes with a length ofmore than 80 mm, the casting properties leave something to be desired.The casting of a perfect, relatively large directionally solidifiedsingle-crystal component from a nickel-base superalloy is extremelydifficult, since most of these components have defects, e.g.,small-angle grain boundaries, freckles, i.e., defects caused by a seriesof identically directed grains with a high eutectic content, equiaxedlimits of variation, microporosity, etc. These defects weaken thecomponents at high temperatures, and consequently the desired servicelife or operating temperature of the turbine are not achieved. However,since a perfectly cast single-crystal component is extremely expensive,the industry tends to permit as many defects as possible without theservice life or operating temperature being adversely affected.

One of the most common defects is grain boundaries, which areparticularly harmful to the high-temperature properties of thesingle-crystal items. Whereas in small components small-angle grainboundaries in relative terms have only a minor influence on theproperties, they are highly relevant to the casting properties andoxidation properties of large SX or DS components at high temperatures.

Grain boundaries are regions with a high local disorder of the crystallattice, since the neighboring grains collide in these regions, andtherefore there is a certain misorientation between the crystallattices. The greater the misorientation, the greater the disorder,i.e., the greater the number of dislocations in the grain boundarieswhich are required for the two grains to fit together. This disorder isdirectly related to the properties of the material at high temperatures.It weakens the material if the temperature rises to above theequicohesive temperature (=0.5×melting point in K).

This effect is known from GB 2 234 521 A. For example, in a conventionalnickel-base single-crystal alloy, at a test temperature of 871° C., thefracture strength drops greatly if the misorientation of the grains isgreater than 6°. This has also been confirmed in single-crystalcomponents with a directionally solidified microstructure, andconsequently the viewpoint has generally been that misorientations ofgreater than 6° are unacceptable.

It is also known from the above-referenced GB 2 234 521 A that enrichingnickel-base superalloys with boron or carbon during a directionalsolidification produces microstructures which have an equiaxed orprismatic grain structure. Carbon and boron strengthen the grainboundaries, since C and B cause the precipitation of carbides andborides at the grain boundaries, and these compounds are stable at hightemperatures. Moreover, the presence of these elements in and along thegrain boundaries reduces the diffusion process, which is a primary causeof the grain boundary weakness. It is therefore possible to increase themisorientations to 10° to 12° yet still achieve good material propertiesat high temperatures. However, these small-angle grain boundaries havean adverse effect on the properties in particular of largesingle-crystal components formed from nickel-base superalloys.

Document EP 1 359 231 A1 describes a nickel-base superalloy which hasimproved casting properties and a higher resistance to oxidation thanknown nickel-base superalloys. Moreover, this alloy is, for example,particularly suitable for large gas turbine single-crystal componentswith a length of >80 mm. It has the following chemical composition(details in % by weight):

7.7-8.3 Cr

5.0-5.25 Co

2.0-2.1 Mo

7.8-8.3 W

5.8-6.1 Ta

4.9-5.1 Al

1.3-1.4 Ti

0.11-0.15 Si

0.11-0.15 Hf

200-750 ppm C

50-400 ppm B

remainder nickel and production-related impurities. However, itscompatibility with TBC (thermal barrier coating) layers, which are usedin particular in the gas turbine sector to protect components exposed toparticularly high thermal stresses, still needs improvement.

SUMMARY

One of numerous aspects of the present invention includes avoiding theabovementioned drawbacks of the prior art. Another aspect includesfurther improving the nickel-base superalloy which is known from EP 1359 231 A1, in particular with a view to achieving better compatibilitywith TBC layers to be applied to this superalloy combined with equallygood casting properties and a high resistance to oxidation compared tothe nickel-base superalloy which is known from EP 1 359 231 A1.

According to yet another aspect of the present invention, a nickel-basesuperalloy embodying principles of the present invention ischaracterized by the following chemical composition (details in % byweight):

7.7-8.3 Cr

5.0-5.25 Co

2.0-2.1 Mo

7.8-8.3 W

5.8-6.1 Ta

4.9-5.1 Al

1.3-1.4 Ti

0.11-0.15 Si

0.11-0.15 Hf

200-750 ppm C

50-400 ppm B

<5 ppm S

5-100 ppm Y and/or 5-100 ppm La

remainder nickel and production-related impurities.

Some advantages of the invention are that the alloy has very goodcasting properties, a high resistance to oxidation at high temperatures,and is very compatible with TBC layers that are to be applied.

It is expedient if the alloy has the following composition (details in %by weight):

7.7-8.3 Cr

5.0-5.25 Co

2.0-2.1 Mo

7.8-8.3 W

5.8-6.1 Ta

4.9-5.1 Al

1.3-1.4 Ti

0.11-0.15 Si

0.11-0.15 Hf

200-300 ppm C

50-100 ppm B

max 2 ppm S

10-80 ppm Y and/or 10-80 ppm La

remainder nickel and production-related impurities.

An advantageous alloy according to the invention has the followingchemical composition (details in % by weight):

7.7 Cr

5.1 Co

2.0 Mo

7.8 W

5.8 Ta

5.0 Al

1.4 Ti

0.12 Si

0.12 Hf

200 ppm C

50 ppm B

1 ppm S

50 ppm Y

10 ppm La

remainder nickel and production-related impurities.

This alloy is eminently suitable for the production of largesingle-crystal components, for example blades or vanes for gas turbines.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The invention is explained in more detail below on the basis of anexemplary embodiment.

Nickel-base superalloys which are known from the prior art (comparisonalloys CA1 to CA5) and the alloy according to the invention A1, havingthe chemical composition listed in Table 1, were tested (details in % byweight): TABLE 1 chemical composition of the alloys tested CA5 (inaccordance CA1 CA2 CA3 CA4 with (CMSX-11B) (CMSX-6) (CMSX-2) (René N5)EP 1359231A) Ni Remainder Remainder Remainder Remainder Remainder A1Remainder Cr 12.4 9.7 7.9 7.12 7.7 7.7 Co 5.7 5.0 4.6 7.4 5.1 5.1 Mo 0.53.0 0.6 1.4 2.0 2.0 W 5.1 — 8.0 4.9 7.8 7.8 Ta 5.18 2.0 6.0 6.5 5.84 5.8Al 3.59 4.81 5.58 6.07 5.0 5.0 Ti 4.18 4.71 0.99 0.03 1.4 1.4 Hf 0.040.05 — 0.17 0.12 0.12 C — — — — 0.02 0.02 B — — — — 0.005 0.005 Si — — —— 0.12 0.12 Nb 0.1 — — — — — Re — — — 2.84 — — S — — — — — 0.0001 Y — —— — — 0.005 La — — — — — 0.001

The alloy A1 is a nickel-base superalloy for single-crystal components,the composition of which is described herein. The alloys CA1, CA2, CA3,CA4 are comparison alloys which are prior art known under thedesignations CMSX-11B, CMSX-6, CMSX-2 and René N5. Inter alia, theydiffer from the alloy according to the invention primarily by virtue ofthe fact that they are not alloyed with C, B, Si, and Y and/or La. Thecomparison alloy CA5 is known from EP 1 359 231 A1 and differs from thealloy according to the invention in terms of the S, Y, and/or Lacontent.

Carbon and boron strengthen the grain boundaries, in particular also thesmall-angle grain boundaries which occur in the <001> direction in SX orDS gas turbine blades or vanes made from nickel-base superalloys, sincethese elements cause the precipitation of carbides and borides at thegrain boundaries, and these compounds are stable at high temperatures.Moreover, the presence of these elements in and along the grainboundaries reduces the diffusion process, which is a primary cause ofthe grain boundary weakness. This considerably improves the castingproperties of long single-crystal components, for example gas turbineblades or vanes with a length of approx. 200 to 230 mm.

The addition of 0.11 to 0.15% by weight of Si, in particular incombination with Hf in approximately the same order of magnitude,significantly improves the resistance to oxidation at high temperaturescompared to the previously known nickel-base superalloys CA1 to CA4.

Restricting the composition according to the invention to a sulfurcontent of <5 ppm produces very good properties, in particular goodbonding of the TBC layer which has been applied to the surface of thesuperalloy, for example by thermal spraying. If the sulfur content is >5ppm, this has an adverse effect on the TBC bonding, and the layerquickly flakes off in the event of fluctuating thermal stresses.

The addition of Y and/or La in the specified range (in each case 5 to100 ppm), i.e., in total, that is to say Y+La, 10 to 200 ppm, if bothelements are present produces very good bonding of the ceramic thermalbarrier coating (TBC layer) which is to be applied.

The Y content of 50 ppm and the La content of 10 ppm specified for thealloy A1 is particularly advantageous, since A1 is particularlycompatible with the TBC layers to be applied. Moreover, these twoelements also increase the resistance to environmental influences. Theaddition of these elements in these low ranges stabilizes thealuminum/chromium oxide scale layer on the alloy surface and produces asignificant resistance to oxidation. Y and La are oxygen-active elementswhich improve the bonding strength of the scale layer on the basematerial. The resistance to spalling during cyclic oxidation is the keyfactor for the stability of the TBC layer.

Table 2 in each case lists the number of cycles which it takes for theAl₂O₃ and other oxide layers formed to flake off under cyclic oxidationat 1050° C./1 h/air cooling to room temperature for the alloys listed inTable 1: TABLE 2 number of cycles until spalling occurs Alloy Number ofcycles until spalling occurs CA1 <30 CA2 200 CA3 80 CA4 230 CA5 1500 A12500

The alloy according to the invention A1, compared to the alloys whichare known from the prior art, has by far the highest number of cyclesbefore the oxide layer flakes off. This implies a high stability of aTBC layer which is to be applied to the surface of the superalloy, forexample by thermal spraying.

If, in other exemplary embodiments, by way of example, nickel-basesuperalloys with higher C and B contents (at most 750 ppm of C and atmost 400 ppm of B) are selected, it is also possible for the componentsproduced from these alloys to be cast conventionally, i.e., without themproducing single crystals.

While the invention has been described in detail with reference toexemplary embodiments thereof, it will be apparent to one skilled in theart that various changes can be made, and equivalents employed, withoutdeparting from the scope of the invention. The foregoing description ofthe preferred embodiments of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Theembodiments were chosen and described in order to explain the principlesof the invention and its practical application to enable one skilled inthe art to utilize the invention in various embodiments as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto, and theirequivalents. The entirety of each of the aforementioned documents isincorporated by reference herein.

1. A nickel-base superalloy having the following chemical composition,in % by weight: 7.7-8.3 Cr 5.0-5.25 Co 2.0-2.1 Mo 7.8-8.3 W 5.8-6.1 Ta4.9-5.1 Al 1.3-1.4 Ti 0.11-0.15 Si 0.11-0.15 Hf 200-750 ppm C 50-400 ppmB <5 ppm S 5-100 ppm Y and/or 5-100 ppm La remainder nickel andproduction-related impurities.
 2. The nickel-base superalloy as claimedin claim 1, having the following chemical composition, in % by weight:7.7-8.3 Cr 5.0-5.25 Co 2.0-2.1 Mo 7.8-8.3 W 5.8-6.1 Ta 4.9-5.1 Al1.3-1.4 Ti 0.11-0.15 Si 0.11-0.15 Hf 200-300 ppm C 50-100 ppm B max. 2ppm S 10-80 ppm Y and/or 10-80 ppm La remainder nickel andproduction-related impurities.
 3. The nickel-base superalloy as claimedin claim 2, having the following chemical composition, in % by weight:7.7 Cr 5.1 Co 2.0 Mo 7.8 W 5.8 Ta 5.0 Al 1.4 Ti 0.12 Si 0.12 Hf 200 ppmC 50 ppm B 1 ppm S 50 ppm Y 10 ppm La remainder nickel andproduction-related impurities.